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Transcript
NMR = Nuclear Magnetic Resonance
Physical Principles:
Some (but not all) nuclei, such as 1H, 13C, 19F, 31P have nuclear spin.
A spinning charge creates a magnetic moment, so these nuclei can be thought of as
tiny magnets.
If we place these nuclei in a magnetic field, they can line up with or against the
field by spinning clockwise or counter clockwise.
N
N
N
S
- spin state,
favorable,
lower energy
S
A spinning nucleus with it's magnetic field
aligned with the magnetic field of a magnet
S
N
- spin state,
unfavorable,
higher energy
S
A spinning nucleus with it's magnetic field
aligned against the magnetic field of a magnet
Alignment with the magnetic field (called ) is lower energy than against the
magnetic field (called ). How much lower it is depends on the strength of the
magnetic field
Note that for nuclei that don’t have spin, such as 12C, there is no difference in
energy between alignments in a magnetic field since they are not magnets. As such,
we can’t do NMR spectroscopy on 12C.
NMR: Basic Experimental Principles
Imagine placing a molecule, for example, CH4, in a magnetic field.
We can probe the energy difference of the - and - state of the protons by
irradiating them with EM radiation of just the right energy.
In a magnet of 7.05 Tesla, it takes EM radiation of about 300 MHz (radio waves).
So, if we bombard the molecule with 300 MHz radio waves, the protons will absorb
that energy and we can measure that absorbance.
In a magnet of 11.75 Tesla, it takes EM radiation of about 500 MHz (stronger
magnet means greater energy difference between the - and - state of the protons)
at no magnetic field,
there is no difference beteen
- and - states.
 proton spin state
(higher energy)
Graphical relationship between
magnetic field (B o) and frequency ( 
E = h x 300 MHz
E
E = h x 500 MHz
for 1 H NMR absorptions
proton spin state
(lower energy)
0T
7.05 T
11.75 T
Bo
But there’s a problem. If two researchers want to compare their data using magnets
of different strengths, they have to adjust for that difference. That’s a pain, so, data
is instead reported using the “chemical shift” scale as described on the next slide.
The Chemical Shift (Also Called ) Scale
Here’s how it works. We decide on a sample we’ll use to standardize our
instruments. We take an NMR of that standard and measure its absorbance
frequency. We then measure the frequency of our sample and subtract its frequency
from that of the standard. We then then divide by the frequency of the standard.
This gives a number called the “chemical shift,” also called , which does not
depend on the magnetic field strength. Why not? Let’s look at two examples.
Imagine that we have a magnet where our standard absorbs at 300,000,000 Hz (300
megahertz), and our sample absorbs at 300,000,300 Hz. The difference is 300 Hz,
so we take 300/300,000,000 = 1/1,000,000 and call that 1 part per million (or 1
PPM). Now lets examine the same sample in a stronger magnetic field where the
reference comes at 500,000,000 Hz, or 500 megahertz. The frequency of our
sample will increase proportionally, and will come at 500,000,500 Hz. The
difference is now 500 Hz, but we divide by 500,000,000 (500/500,000,000 =
1/1,000,000, = 1 PPM).
It’s brilliant.
Of course, we don’t do any of this, it’s all done automatically by the NMR machine.
Even more brilliant.
The Chemical Shift of Different Protons
NMR would not be very valuable if all protons absorbed at the same frequency.
You’d see a signal that indicates the presence of hydrogens in your sample, but any
fool knows there’s hydrogen in organic molecules. What makes it useful is that
different protons usually appear at different chemical shifts (. So, we can
distinguish one kind of proton from another. Why do different protons appear at
different ? There are several reasons, one of which is shielding. The electrons in a
bond shield the nuclei from the magnetic field. So, if there is more electron density
around a proton, it sees a slightly lower magnetic field, less electron density means
it sees a higher magnetic field:
Z
C
H
This represents the electron density of a C-H bond. How much electron
density is on the proton depends on what else is attached to the carbon. If Z
is an elelctronegative atom, the carbon becomes electron deficient and pulls
some of the electron density away from the H. if Z is an electron donating
group, more electron density ends up on the H.
How do the electrons shield the magnetic field? By moving. A moving charge
creates a magnetic field, and the field created by the moving electrons opposes the
magnetic field of our NMR machine. It’s not a huge effect, but it’s enough to
enable us to distinguish between different protons in our sample.
The Hard Part - Interpreting Spectra
Learning how an NMR machine works is straightforward. What is less
straightforward is learning how to use the data we get from an NMR machine (the
spectrum of ethyl acetate is shown below). That’s because each NMR spectrum is a
puzzle, and there’s no single fact that you simply have to memorize to solve these
spectra. You have to consider lots of pieces of data and come up with a structure that
fits all the data. What kinds of data do we get from NMR spectra? For 1H NMR,
there are three kinds each of which we will consider each of these separately:
1) Chemical shift data - tells us what kinds of protons we have.
2) Integrals - tells us the ratio of each kind of proton in our sample.
3) 1H - 1H coupling - tells us about protons that are near other protons.
Chemical Shift Data
As previously mentioned, different kinds of protons typically come at different
chemical shifts. Shown below is a chart of where some common kinds of protons
appear in the  scale. Note that most protons appear between 0 and 10 ppm. The
reference, tetramethylsilane (TMS) appears at 0 ppm, and aldehydes appear near 10
ppm. There is a page in your lab handout with more precise values for this chart.
Note that these are typical values and that there are lots of exceptions!
R
R
NH
OH
R
Ph
Me
OH
(R)
HO CH3
R
TMS = Me
Ph CH3
R
R
O
O
R
NR2 O
Cl
H
H
H
R
OCH3
H
CH3
CH3
Si Me
Me
R
CH3
CH3
TMS
10
9
Downfield region
of the spectrum
8
7
6
5
ppm
4
3
2
1
Upfield region
of the spectrum
0
Integrals
Integrals tell us the ratio of each kind of proton. They are lines, the heights of which
are proportional to the intensity of the signal. Consider ethyl acetate. There are
three kinds of protons in this molecule, the CH3 next to the carbonyl, the CH2 next to
the O and the CH3 next to the CH2. The ratio of the signals arising from each of
these kinds of protons should be 3 to 2 to 3, respectively. So, if we look at the height
of the integrals they should be 3 to 2 to 3. With this information, we can know
which is the CH2 signal (it’s the smallest one), but to distinguish the other two, we
have to be able to predict their chemical shifts. The chart on the previous page
allows us to make that assignment (the CH3 next to the C=O should appear at ~ 2
PPM, while the other CH3 should be at ~ 1 PPM).
3H'S
O
O
CH3
O
O
O
H
H 3C
H
3H'S
2 H'S
O
1H
- 1H Coupling
You’ll notice in the spectra that we’ve seen that the signals don’t appear as single
lines, sometimes they appear as multiple lines. This is due to 1H - 1H coupling (also
called spin-spin splitting or J-coupling). Here’s how it works: Imagine we have a
molecule which contains a proton (let’s call it HA) attached to a carbon, and that this
carbon is attached to another carbon which also contains a proton (let’s call it HB). It
turns out that HA feels the presence of HB. Recall that these protons are tiny little
magnets, that can be oriented either with or against the magnetic field of the NMR
machine. When the field created by HB reinforces the magnetic field of the NMR
machine (B0 ) HA feels a slightly stronger field, but when the field created by HB
opposes B0, HA feels a slightly weaker field. So, we see two signals for HA
depending on the alignment of HB. The same is true for HB, it can feel either a
slightly stronger or weaker field due to HA’s presence. So, rather than see a single
line for each of these protons, we see two lines for each.
For this line, H B is lined up
with the magnetic field
(adds to the overall
magnetic field, so the line
comes at higher frequency)
For this line, H B is lined up
against the magnetic field
(subtracts from the overall
magnetic field, so the line
comes at lower frequency)
HA
HA is split into two lines because
it feels the magnetic field of H B.
HB
HB is split into two lines because
it feels the magnetic field of H A.
HA
HB
C
C
More 1H - 1H Coupling
What happens when there is more than one proton splitting a neighboring proton?
We get more lines. Consider the molecule below where we have two protons on one
carbon and one proton on another.
Note that the signal produced
by HA + HA' is twice the size
of that produced by H B
HA + HA'
HA and H A' appear at the same
chemical shift because they are
in identical environments
They are also split into two lines
(called a doublet) because they
feel the magnetic field of H B.
HB
HB is split into three lines
because it feels the magnetic
field of H A and HA'
HA'
HA
HB
C
C
Why are There Three Lines for HB?
HB feels the splitting of both HA and HA’. So, let’s imagine starting with HB as a
single line, then let’s “turn on” the coupling from HA and HA’ one at a time:
HB
If uncoupled, H B would appear as a
singlet where the dashed line indicates
the chemical shift of the singlet.
Now, let's "turn on" HB - HA coupling. This splits
the single line into two lines
Now, let's "turn on" HB - HA' coupling. This
splits each of the two new lines into two lines,
but notice how the two lines in the middle
overlap. Overall, we then have three lines.
HA'
HA
HB
C
C
Because the two lines in the middle overlap, that line is twice as big as the lines on
the outside. More neighboring protons leads to more lines as shown on the next
slide.
Splitting Patterns with Multiple Neighboring Protons
If a proton has n neighboring protons that are equivalent, that proton will be split
into n+1 lines. So, if we have four equivalent neighbors, we will have five lines, six
equivalent neighbors… well, you can do the math. The lines will not be of equal
intensity, rather their intensity will be given by Pascal’s triangle as shown below.
no. of neighbors
relative intensities
pattern
0
1
singlet (s)
1
2
3
1
1
1
1
2
3
example
doublet (d)
1
H
H
C
C
triplet (t)
3 1
quartet (q)
H
H
C
C
H
H
C
C
H
H
4
5
1 4
6 4
1 5 10 10
1
5 1
pentet
sextet
H
H
H
H
C
C
C
1 6 15 20 15 6 1
septet
H
H
H
C
C
C
H
H
H
H
6
H
H
H
H
H
C
C
C
H
H
H
We keep emphasizing that this pattern only holds for when the neighboring protons
are equivalent. Why is that? The answer is two slides away.
More About Coupling
Earlier we said that protons couple to each other because they feel the magnetic
field of the neighboring protons. While this is true, the mechanism by which they
feel this field is complicated and is beyond the scope of this class (they don’t just
feel it through space, it’s transmitted through the electrons in the bonds). It turns
out that when two protons appear at the same chemical shift, they do not split each
other. So, in EtBr, we have a CH3 next to a CH2, and each proton of the CH3
group is only coupled to the protons of the CH2 group, not the other CH3 protons
because all the CH3 protons come at the same chemical shift.
The blue protons all come
at the same chemical shift
and do not split each other
H H
H C
C
H H
Br
The red protons both come
at the same chemical shift
and do not split each other
H H
H H
H C
C
H H
H C
Br
C
H H
Br
Not all Couplings are Equal
When protons couple to each other, they do so with a certain intensity. This is
called the “coupling constant.” Coupling constants can vary from 0 Hz (which
means that the protons are not coupled, even though they are neighbors) to 16 Hz.
Typically, they are around 7 Hz, but many molecules contain coupling constants that
vary significantly from that. So, what happens when a molecule contains a proton
which is coupled to two different protons with different coupling constants? We get
a different pattern as described in the diagram below.
So, if the protons are not equivalent, they can have different coupling constants and
the resulting pattern will not be a triplet, but a “doublet of doublets.” Sometimes,
nonequivalent protons can be on the same carbon as described on the next slide.
Coupling Constants in Alkenes
Coupling constants in alkenes can also differ depending on whether the protons are
cis or trans to each other. Note that in a terminal alkene (i.e., an alkene at the end of
a carbon chain), the cis and trans protons are NOT equivalent. One is on the same
side as the substituent, the other is on the opposite side. The coupling of trans
protons to each other is typically very large, around 16 Hz, while the coupling of cis
protons, while still large, is a little smaller, around 12 Hz. This leads to the pattern
shown below, and an example of a molecule with this splitting pattern is shown on
the next slide.
H
If uncoupled, H would appear as a
A
12Hz coupling
HA
HM
16 Hz
12 Hz
HX
16 Hz coupling
A
singlet where the dashed line indicates
Now, let's "turn on" HA - HX coupling. This splits
the single line into two lines that are 16 Hz appart
12 Hz
Now, let's "turn on" HA - HM coupling. This
splits each of the two new lines into two lines
that are 12 Hz appart for a total of four lines
There are other times when protons on the same carbon are nonequivalent, which
we’ll see later.
HO
H
HO
A molecule with a terminal alkene
CH3
H
H
HO
HO
H
H HO
CH3
H
H
H
H HO
HO
H
H
A molecule with a nine line splitting pattern
Me
OH
Me
Nine lines, you just can't
see two of them because
they are so small.
Me
OH
Me
H
H
Me
Me
OH
H
Me
Me
OH
OH